The present invention relates to a method of controlling the molecular weight distribution of a vapor. Particularly, the invention is directed to a method to control the composition of a lubricant vapor, the lubricant having multiple components with different vapor pressures. The invention is also directed to an apparatus used to control the molecular weight distribution of a vapor.
Magnetic discs with magnetizable media are used for data storage in most all computer systems. Current magnetic hard disc drives operate with the read-write heads only a few nanometers above the disc surface and at rather high speeds, typically a few meters per second. Because the read-write heads can contact the disc surface during operation, a thin layer of lubricant is coated on the disc surface to reduce wear and friction.
Lubricants conventionally employed in manufacturing magnetic recording media typically comprise mixtures of long chain polymers characterized by a wide distribution of molecular weights and include perfluoropolyethers, functionalized perfluoropolyethers, perfluoropolyalkylethers (PFPE), and functionalized PFPE. PFPE do not have a flash point and they can be vaporized and condensed without excessive thermal degradation and without forming solid breakdown products. The most widely used class of lubricants include perfluoropolyethers such as AM 2001(copyright) or Z-DOL(copyright).
During a lubricant vapor lubrication process, the lubricant""s vapor phase molecular weight distribution is dependent on the liquid phase distribution and the liquid phase temperature. As the lighter fractions of a polydisperse liquid lubricant are preferentially evaporated, over time the composition of both the liquid phase and vapor phase lubricant is changing. This phenomenon is demonstrated in FIG. 1, which shows lubricant molecular weight distributions in the vapor phase versus the liquid phase distribution, over four days of continuous vaporization at typical production deposition rates. The shift in vapor phase molecular weight over time results in a non-constant and uncontrolled process from disc to disc.
The molecular weight of a lubricant affects the mechanical performance of the interface and, as a consequence, optimal ranges of molecular weight distribution are typically determined for a particular lubricant. Generally, the lubricant is applied to the disc surface by dipping the disc in a bath containing the lubricant. The bath typically contains the lubricant and a coating solvent to improve the coating characteristics of the lubricant which is usually a viscous oil. The discs are removed from the bath, and the solvent is allowed to evaporate, leaving a thin layer of lubricant on the disc surface. However, because the solvent is usually released to the atmosphere, it may raise certain present and future environmental issues. Therefore, an improved process to apply lubricants to recording media without the use of coating solvents is needed.
The invention utilizes a vapor deposition process to apply lubricants to recording media. The invention provides a substantially steady-state concentration of one or more molecular components of the lubricant to a vapor deposition chamber.
One embodiment is a method for providing a lubricant vapor in a process for manufacturing recording media comprising, adding a lubricant to a device, the lubricant having a plurality of molecular components and a weight average molecular weight Ml, each of the molecular components having an associated vapor pressure; vaporizing the lubricant to form the lubricant vapor at a vaporization rate such that the lubricant vapor has a weight average molecular weight Mv, wherein the ratio Mv:Ml is from 0.3 to 1 during the process for manufacturing recording media. The method could further comprise depositing the lubricant vapor on a recording medium.
The vaporizing of the lubricant could comprise maintaining a Mv:Ml ratio in a range selected from the group consisting of 0.3 to 0.98, 0.5 to 0.95, and 0.7 to 0.95. The lubricant could be a solid lubricant, a liquid lubricant or mixtures thereof, e.g., polyfluoropolyether. The lubricant could have a Mw:Mn ratio between 1 and 1.6. The lubricant could comprise 70% by weight molecular components having a molecular weight selected from the range consisting of 1 kD to 10 kD, 2 kD to 8 kD, 1 kD to 8 kD, 1 kD to 4 kD, and 2 kD to 4 kD.
In another embodiment, the vaporizing the lubricant could comprise maintaining a vapor phase comprising the lubricant vapor having a weight average molecular weight Mv which does not vary by more than Z percent during the process for manufacturing recording media, wherein Z percent is selected from the group consisting of 10%, 20%, 30%, 40% and 50%.
One embodiment further could comprise providing a carrier gas to transport the lubricant vapor to a recording medium positioned within a vapor deposition chamber.
The device of this invention could comprise two or more heated reservoirs to vaporize the lubricant. The lubricant could be directed from one reservoir maintained at a first temperature to another reservoir maintained at a second temperature. The device could comprise a programmer to control the first temperature and the second temperature and the rate at which the lubricant flows from one reservoir to another reservoir.
One embodiment could comprise providing a line of sight deposition of the lubricant vapor onto a surface of the recording medium. Another embodiment could comprise maintaining the recording medium at a temperature to allow selective condensation of higher molecular weight components from the lubricant vapor.
Another embodiment is a method for providing a lubricant vapor, comprising adding a lubricant to a device, vaporizing the lubricant to form the lubricant vapor, wherein the vaporizing the lubricant comprises maintaining a vapor phase comprising the lubricant vapor having a weight average molecular weight Mv which does not vary by more than Z percent during the process for manufacturing recording media, wherein Z percent is selected from the group consisting of 10%, 20%, 30%, 40% and 50%.
Another embodiment is an apparatus for providing a lubricant vapor in a process for manufacturing recording media, comprising means for adding a lubricant to a device, the lubricant having a plurality of molecular components and a weight average molecular weight Ml, each of the molecular components having an associated vapor pressure; and means for vaporizing the lubricant to from a lubricant vapor at a vaporization rate such that the lubricant vapor has a weight average molecular weight Mv, wherein the ratio Mv:Ml is from 0.3 to 1 during the process for manufacturing recording media. The means for adding a lubricant could comprise an injector for drop-wise addition of the lubricant to the apparatus for providing the lubricant vapor or a pump for pumping the lubricant into the apparatus. The means for vaporizing the lubricant to form the lubricant vapor could comprise multiple heated reservoirs or a porous tube with multiple heating zones.
In the apparatus, the means of vaporizing the lubricant at a vaporization rate could comprise maintaining a vapor phase comprising the lubricant vapor having a weight average molecular weight Mv which does not vary by more than Z percent during the process for manufacturing recording media, wherein Z percent is selected from the group consisting of 10%, 20%, 30%, 40% and 50%.
The device for controlled vaporization of the lubricant could comprise a porous tube that is variably heated along its length with one or more thermal sources, the lubricant vapor passing through pores on a surface of the tube into a chamber. The porous tube could contain two or more heating zones along its length, the lubricant vapor passing through pores on a surface of the tube into a chamber.
In another embodiment, the device could comprise an injector that adds lubricant to the device such that a substantial proportion of the lubricant is vaporized on contact with a heating zone. The device could comprise an injection nozzle to convert the lubricant to an aerosol.